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Abstract
An in-fiber integrated quasi-distributed high temperature sensor array is proposed and demonstrated. The sensor array consists of some weakly reflective joint surfaces which are welded by single mode fiber (SMF) and double-clad fiber (DCF). The characteristics of the reflected signal of the sensor array are analyzed, and the relationship between the signal intensity and the number of sensors is simulated for evaluating sensor multiplex capacity. Due to its all-silica structure, the sensor array could test temperature up to 1000°C for a long time. This sensor array is flexible and easy to be fabricated only by splicing without any connector, which will be beneficial to space constrained quasi-distributed high temperature sensing applications.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The modern industry and the energy industry often include some production process with harsh environmental conditions, just like high temperature and high pressure [1,2]. Few sensors are able to provide accurate and reliable information during these processes. To solve this problem, new sensor technologies are urgently needed. Compared to other sensors, optical fiber sensor has lots of advantages, such as low cost, high flexibility, good electro-magnetic immunity characteristic and others. Without coating, optical fiber has a good high-temperature resistance characteristic because the material is SiO2 which can withstand the temperature as high as 1600°C. However, when the temperature is higher than 1300°C, the thermal diffusion is significant and the refractive index profile of the fiber will be changed [3,4]. In spite of this, optical fiber can keep stable below 1300°C which makes it become a very suitable candidate for high temperature sensing. In recent researches, optical fiber Fabry–Perot (FP) sensor has been widely used in high temperature sensing [5–8], such as a section of single mode fiber (SMF) spliced with a section of special fiber to form a FP tip sensor [5–7], a micro cavity FP sensor formed by an ultra-thin film embedded in a continuous fiber [8]. Fiber Bragg gratings (FBGs) are widely used in high temperature sensing [9–13]. A high temperature (1100°C) sensor based on surface relief FBGs has been reported and demonstrated [9]. Warren-Smith fabricated gratings on suspended-core micro-structured optical fibers to measure the temperature up to 1300°C [11]. Recently, it is common to see that sapphire optical fiber has been used for harsh environment sensing due to its high melt temperature [10,12–14]. The interference effect of multicore fiber [15,16] and multimode interference effect of special fiber [17,18] are also widely studied for high temperature sensing. Such sensors usually have high sensitivity and accuracy. However, these sensors also have some limitations. For instance, they usually need an optical spectrum analyzer to demodulate the signal. This is bound to increase the complexity and cost of the sensing system. Moreover, it is difficult to measure the multipoint temperature simultaneously due to the high insertion loss.
High temperature sensing is a difficult task for the vast majority of sensors. And it is even more difficult to achieve distributed high temperature sensing measurement. A multipoint sensor using sapphire fiber based on air gap was presented by Wang [19]. They implemented three sensors in series and measured the temperature simultaneously, and the temperature was above 1000°C. However, the high reflection of the air gap limits the number of sensors in sensor array. In 2018, Yan proposed multi-point fiber optic sensors based on multi-mode random air hole fibers infiltrated with CdSe/ZnS quantum dots [20]. In the paper, each test point requires a multimode fiber sensor, which makes the sensor complex, not easy to integrate, and unable to be used in a volume constrained environment. To solve these problems, we propose and demonstrate a novel in-fiber quasi-distributed high temperature sensor array. By using this sensor array, we achieved a quasi-distributed temperature measurement up to 1000°C. Moreover, since all the sensor integrated in a fiber, the high temperature sensor array is more suitable for the circumstances where the size of the environment is limited.
2. Sensor fabrication and theoretical analysis
2.1 The structure of the sensor array and the fabrication procedure
The configuration of the proposed in-fiber quasi-distributed high temperature sensor array is shown in Fig. 1. The sensor array consists of SMF and double-clad fiber (DCF) welded alternatively. The SMF and the DCF are spliced by a fiber fusion splicer to construct a weakly reflective joint surface.
Figures 2(a) and 2(b) are the cross-section images of the DCF and SMF, respectively. We tested the refractive index distribution of the two kinds of fibers by a refractive index analyzer (Photon Kinetics S14). The results are shown in Figs. 2(c) and 2(d). There is a difference between the two claddings of the DCF. The refractive indexes of the inner cladding and outer cladding are about 1.460 and 1.457, respectively. And the refractive index of the SMF is shown in Fig. 2(d). The refractive index of the SMF core is almost the same as the inner cladding of the DCF. Therefore, there is a refractive index difference between the two fiber cores. From Fresnel Law [21], the reflectivity of the fusion joint could be calculated by
where n1 and n2 are the refractive indexes of the core of the two kinds of fibers, respectively. The cores of the DCF and the SMF have large refractive index difference, so that the joint surface between the two fibers has a high reflectivity. Therefore, the DCF is a very suitable candidate for the sensor array.
Fig. 2 The images captured by microscope and refractive index analyzer. (a,b) The cross-image of the DCF and the SMF, (c,d) the refractive index profile of the DCF and the SMF.
The fabrication process of the in-fiber sensor array is as follows. First, we used a fiber fusion splicer (NT-400F) to weld the DCF and the SMF. Then cutting the fiber at a certain length. After that, splicing another fiber to make the DCF and the SMF connected alternately. Repeating the above procedure several times to make up the sensor array.
2.2 Analysis and stimulation of the sensor array
Because of the refractive index difference between the two fiber cores, a weakly reflective joint surface is formed as mentioned above. As mentioned above, the reflectivity of the joint surface depends on the refractive index difference between the two fiber cores. And it is generally known that the refractive index profile of the fiber core is determined by the concentration of germanium ions (Ge) doped in SiO2. However, owing to the thermal diffusion effect [3,4,22], the Ge doped in the fiber core will spread into the cladding when the fiber is placed in a high temperature condition. The concentration of Ge will decrease with the time, which leads to the decrease of the refractive index of the fiber core. In addition, the refractive index difference between the two fiber cores become smaller and the reflectivity will decrease, too. In our fabrication process, longer splicing time results in lower reflectivity of the reflective surface. However, the insertion loss of the joint decreases with the splicing time increasing. The insertion loss determines the multiplexable capability of the sensor array. So that, the splicing time is a paradox for the insertion loss and the reflectivity of the joint surface.
In Figs. 3(a) and 3(b), we tested the relationship between the splicing time, the insertion loss and the reflectivity of the joint surface, respectively. To test the reflectivity of the joint surface, we used an optical low-coherence domain reflectometry (OLCR). And an optical power meter was used to test the insertion loss. In Figs. 3(a) and 3(b), the results are average values of ten measurements. In Figs. 3(a) and 3(b), the reflected intensity and insertion loss of the joint surface both decreases with the splicing time increasing. The insertion loss decrease means more sensors can be connected in sensor array. Contrarily, the signal to noise ration (SNR) comes down due to the reduction of the reflected intensity, which degrades the multiplexable capability of sensor array. Taking these two factors into account, we chose 500 ms as the experimental splicing time.
Fig. 3 (a,b) The relationship of the fusion splicing time versus the joint reflectivity and insertion loss, (c) the relationship between the normalized output power of sensor array and the number of sensors in different splicing time, (d) the maximum number of sensors in series versus splicing time.
The simulation results of the multiplexable capability of the sensor array is shown in Fig. 3(b). Supposing the input light intensity is IT(0) as shown in Fig. 1, and the insertion loss and the reflectivity of the kth sensor are ηk and Rk, respectively. The reflected light and the transmitted light intensity of the kth sensor are calculated as following:
From Eq. (3), the transmitted light intensity is affected by the insertion loss and the reflectivity of the joint surface. Because the reflectivity is small (about 10−6~10−7), the insertion loss has a greater impact on the light transmission. We simulated the relationship of the maximum number of sensors in series versus the splicing time. Firstly, we simulated the the normalized intensity of transmitted light by changing the splicing time, and the results is shown in Fig. 3(c). Secondly, the identification level is set to 10%, then the maximum number of sensors in sensor array at different splicing time can be obtained as shown in Fig. 3(d).3. Temperature measuring principle
The configuration of the experimental setup is shown in Fig. 4. The system consists of an ASE source, a 4-port optical fiber circulator, an optical fiber collimator, a scanning mirror, a PD and a data acquisition (DAQ) card. The collimator is coated by a transflective film whose reflectivity is about 38%, so that the intensities of the two reflected lights (the red and blue line in Fig. 4) are almost the same. These devices form a white light interference demodulation interferometer (WLIDI). Then the sensor array is demodulated by the WLIDI to get the temperature information. The illustration of the optical path correlation is shown in Fig. 4. Taking the kth sensor as an example. With the scan of the mirror, there is an optical path difference 2Xk between the two reflected lights. The two reflected lights transmit into the sensor array. Until the two lights reach the kth sensor. One light is reflected by the right-hand surface of the sensor, and the other light is reflected by the left-hand surface of the sensor. Supposing the length of the kth sensor is lk. The optical path correlation condition is met when 2Xk = 2nklk, that is
where nk is the refractive index of the kth fiber sensor.Fig. 4 The configuration of the experimental setup. The OPD is the Optical path difference, and the OPC is the optical path compensation. The optical path correlation condition is satisfied when the OPD is equal to the OPC.
The nklk is the optical path (OP) of the kth sensor,
Generally, the OP is a function of the temperature and the strain. Assume that the strain applied on the fiber is constant. Then the change of optical path owing to the temperature changes can be expressed as
where αT and CT are the thermal expansion coefficient and thermo-optic coefficient, respectively. αT = 0.055*10−5/°C and CT = 0.811*10−5/°C, taken from [23,24].The temperature variety cause the OP changes of the cascade array sensor formed by the weakly reflecting surface [24,25]. Then the temperature variety could be sensed by
T0k is the initial ambient temperature of the kth sensor. Sk = nklk(αT ± CT) is the kth sensor temperature sensitivity. It is related to the length of the fiber sensor and the material properties, such as refractive index, thermal expansion coefficient and thermo-optic coefficient. Therefore, the temperature sensitivity could be appropriately adjusted by changing the length of the sensor.
4. Experimental results
4.1 Temperature calibration experiment
In the temperature calibration experiment, we used the WLIDI to demodulate the sensor array. We chose two fiber sensors, a SMF and a DCF sensor which compose the sensor array. The length of the DCF and SMF sensors are 115 mm and 100 mm, respectively. In order to calibrate the temperature of the sensor array, it was placed into a micro tube furnace (Chengyi: CHY-1200) with a maximum temperature of 1200°C. The temperature rose from ambient temperature to 1000°C and held for twenty minutes. After the micro tube furnace temperature stabilized, we began the temperature test and start the cooling process. The temperature was reduced by 10°C per minute. Figure 5 shows the temperature calibration result for the two sensors. A linear relation TSystem output temperature = K∙T is got from the result. The response ratios of the sensors are 1.1354 and 1.3133 for SMF sensor and DCF sensor, respectively. The two sensor data fitting degrees are 0.9955 and 0.9957, respectively. From Fig. 5, when the test temperature is below 200°C, the linearity of the curve is poor. The reason probably is that the ends of the tubular furnace are open, so the real temperature for the fiber sensor is different from the thermocouple in the tubular furnace.
4.2 Quasi-distributed temperature sensing experiment
The experimental setup of quasi-distributed temperature sensing is shown in Fig. 6(a). A SiC heating rod was used in our quasi-distributed temperature sensing test. The geometry size of the SiC is as follows: the diameter of the heating rod is 14 mm, and the total length is 400 mm. The two ends of the heating rod are two electrodes for loading voltage. The middle part of the SiC is about 200 mm and it is composed of three parts: the heating zone is sandwiched between two temperature transition zones. Figure 6(b) shows the heating rod after heating and its temperature distribution. A thermocouple was fixed on a translation stage to measure the temperature distribution of the heating rod. We fixed the fiber sensor array on the surface of the SiC heating rod as shown in Fig. 6(a). The sensor array is composed by five sensors, including three DCF sensors (S1, S3 and S5) and two SMF sensors (S2 and S4). The lengths of the sensors are 40.0 mm, 41.6 mm, 44.8 mm, 45.6 mm, 48.2 mm, respectively. The scanning results of the sensor array before and after heating scanned by the WLIDI are shown in Fig. 7(a).
Fig. 6 (a) The experimental setup of quasi-distributed high temperature sensing, (b) the SiC heating rod after heating and its temperature distribution.
Fig. 7 (a) The scanning output of the sensor array scanned by the WLIDI before and after heating, (b) the single sensor S3 trend with temperature.
The temperature of the SiC increased with the increase of the voltage applied to it. We gradually adjusted the voltage applied across the heating rod. When the voltage was 16 V, 22 V, 30 V and 38 V, we tested the temperature distribution of the heating rod by the thermocouple. Meanwhile, we used the WLIDI to scan the optical fiber sensor array. Figure 7(b) shows the trend of the S3 with the temperature. Then there are four curves from the thermocouple in Fig. 8, which are four temperature distributions of the SiC rod, respectively. The scatter plots are measured by the optical fiber sensor array. And each scatter point is the temperature average of the area where the sensor measures. The temperature trend of the sensor array is consistent with the test result of the thermocouple.
Fig. 8 The results of the quasi-distributed temperature sensing by the sensor array and thermocouple. The solid lines are measured by the thermocouple at the different condition, and the scatter plots are measured by the fiber sensor array. The position relationship between the SiC and the fiber sensor array is shown in Fig. 6.
4.3 Temperature stability experiment
In order to test the robust and stability of the sensor array, the joint surface spliced by the DCF and the SMF was placed in a ceramic micro heater (NTT Advanced technology Corporation) for a long time. The temperature was measured by the fiber sensor. And the result is shown in Fig. 9. There are four temperature curves in the picture, which are the test results of the reflectivity of the joint surface changing with time at four temperatures. The reflectivity almost has no changes at 1100°C within two hours. Along with the rise of temperature, the reflectivity gradually decreases, and the higher temperature means the faster rate of decline. The reason is as mentioned in the second section. The thermal diffusion effect of the Ge doped in the fiber core will be more obviously with the higher temperature. And it makes the reflectivity of the reflective surface gradually decrease. The robust and stability are the advantages of the sensor array when the temperature is below 1200°C. From [3], the refractive index of the optical fiber core changes very little in 10 hours at 1200°C.
5. Discussion and conclusions
In this paper, we demonstrated quasi-distributed high temperature sensing up to 1000°C using an in-fiber integrated white light interference sensor array. The reflectivity of the joint surface between the two fibers in the sensor array remains virtually unchanged at 1100°C for two hours. And the reflectivity is decrease obviously when the temperature higher than 1200°C due to the the thermal diffusion [3,4]. Future research needs to focus on the package of the sensor array. Without coating, the sensor array is easily broken and difficult to be widely used. After overcoming the packaging problem, the sensor array has a great potential to be used in quasi-distributed high temperature sensing application, such as measuring the temperature distribution inside the motor, machine tool, and other high temperature devices.
Funding
National Natural Science Foundation of China (NSFC) (61535004, 61735009, 61827819); Guangxi project (AD17195074); National Defense Pre-Research Foundation of China (CN) (6140414030102).
Acknowledgments
The authors would like to thank Shitai Yang and Gongdai Chen for the discussion.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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